1. Field of Invention
The invention generally relates to a semiconductor device. More particularly, the invention relates to a method for forming a gate in a semiconductor device.
2. Brief Description of Related Technology
As a design rule of a semiconductor device is decreased, a numerical aperture and a wavelength (λ) of a light source are increased to form a fine pattern. The exposure condition is advantageous in a cell region during a gate formation, but it is difficult to secure a depth of focus (“DOF”) margin in a peripheral region so that there is a limitation on regulation of a gate critical dimension (“CD”). A trimming process is performed while the peripheral region is exposed, thereby independently controlling the gate CD over the cell region.
It is necessary to control the CD within 1 nanometer (nm) to pattern a CD of 45 nm, which is the minimum gate CD in devices beyond 65 nm. As a result, a photoresist trimming process is required. The trimming process reduces a width of the photoresist film by a dry etching method to define a desired width of a fine pattern.
However, because it is difficult to apply the trimming process in practice, the gate CD is adjusted with a mask. The mask includes an amorphous carbon material to improve durability and minimize loss resulting from a strip process.